Patch antennas are very popular in telecommunication devices since they are rugged and are easily incorporated into the device. A patch antenna consists of a metal patch plate that is suspended over a metal ground plane. Generally the metal patch plate is essentially rectangular with an even or almost even width and length. The ground plate is generally the same size or slightly larger to provide optimal signal reception.
Typically, a patch antenna with a vacuum between the plates has a resonant frequency for a signal when the patch plate has a length of about half the size of the wavelength of the signal, for example for a GPS signal with a frequency of approximately 1.5-1.6 GHz, half a wavelength would be about 95 mm. When manufacturing miniature devices a smaller patch antenna is desired. The use of a dielectric material between the plates reduces the required plate size. The required length for a patch antenna with a dielectric material between the plates can be calculated by the equation: L=C/2f(∈)1/2. Wherein L=the patch size length, C=the speed of light, f=the frequency of the signal under consideration, and ∈ is the dielectric constant. In the example given above the use of a dielectric with a dielectric constant of 17 would provide a length value of approximately 23 mm, which can be incorporated more readily into a miniature device.
The distance between the plates of the antenna also influences the effective bandwidth of the antenna, generally the smaller the distance between the plates the less the energy is radiated and the more the energy is stored in the patch antenna as capacitance and inductance. On the other hand too big a distance also reduces the effectiveness of the antenna. Thus there is generally an optimal distance for positioning the plates; however the height may be dictated by size considerations of the device in which the antenna is to be used. The effectiveness of the antenna is generally measured by the Q factor (quality factor). A low Q factor signifies a high rate of energy loss and a low gain, whereas a high Q factor signifies a low rate of energy loss and a high gain. Some people refer to the dissipation factor (DF), which is proportional to the inverse of the Q factor, to represent the effectiveness of the antenna. As an example the Web site www.emtalk.com/mpcalc.php provides a calculator for calculating the required size (length, width) of a patch antenna based on a given resonant frequency, a dielectric constant and dielectric height.
The dielectric material used between the plates also affects the gain provided by a specific antenna configuration. Generally, the higher the dielectric constant of the dielectric used between the plates the greater the energy loss and the lower the gain, thus by introducing a dielectric material with a high dielectric constant to reduce the size required for the antenna we also reduce the signal gain provided by the antenna. In spite of this general rule some dielectric materials have a lower dissipation factor than others even if they have approximately the same dielectric constant.
By defining an antenna size and specific resonant frequency we define the required dielectric constant. Generally, certain materials with specific dielectric constants are available in the market; however dielectric manufacturers can tailor to the needs of clients by preparing a dielectric material with a specific dielectric constant by mixing dielectric materials.